HIV transmission can be decreased substantially by reducing the proportion of undiagnosed infections and expanding early and consistent use of antiretroviral therapy (ART). In fact, Treatment as Prevention (TasP) has been proposed as key to ending the HIV epidemic. To activate TasP in high prevalence countries, like South Africa, communities must be motivated to know their status, engage in care, and remain in care - community mobilization (CM) is needed. CM has the potential to significantly increase uptake testing, linkage to and retention in care, essential to population level declines in HIV by addressing the primary social barriers to engagement with the HIV care system. CM strategies for linkage and retention have not yet been rigorously evaluated despite their promise. To address this gap, we aim to implement and evaluate a theory-based CM intervention addressing known social barriers to engagement in care with the aim of increasing HIV testing uptake, thereby decreasing undiagnosed infections (aims 1), improving linkage to care (aim 2), and increasing retention in care (aim 3) in rural South Africa, while also exploring the mechanisms through which CM improves outcomes along the care cascade (aim 4). The CM intervention will be evaluated by comparing gains in testing, linkage, and retention in care in 16 intervention nave communities (8 intervention and 8 control) within a health and demographic surveillance site (HDSS) in rural Mpumalanga province, South Africa. The CM intervention is based on four years of formative social science research and two years of implementation experience in the area utilizing the same conceptual model developed by our team. The intervention activities map onto six domains of CM that must be addressed for CM to successfully occur: 1) development of a shared concern around TasP; 2) community sensitization or building of critical consciousness; 3) an organizational structure to links to groups/networks in supporting TasP; 4) leadership; 5) collective activities/actions; and 6) community cohesion. Mobilization activities include pilot-tested strategies and will be carried out by our implementing partner, Sonke Gender Justice. To measure rates of testing, linkage and retention, we will link an existing electronic clinical tracking system in use in all clinics in the HDSS to the longitudinal HDSS census data. This provides a unique opportunity to establish an open, population-based cohort with over 30,000 18-49 year old residents. Merging census and clinical data allows us to simultaneously evaluate the effects of our intervention and to characterize the HIV care cascade in a high prevalence population. In addition, we will evaluate CM processes by conducting community based surveys to monitor intervention exposure and community level change using validated measures of CM. This proposal builds on our teams experience with CM in the study area and a strong team of partners-we are well positioned to conduct a rigorously designed and evaluated trial to determine the impact of CM on the care cascade, helping to fill an urgent need.